Miscellaneous compounding processes
Melt blenders having counter- or corotating rotors enclosed in a heated chamber are employed to melt blend the fiber—matrix mixture prior to injection or compression molding. A handful of studies have also reported the use of high speed mixers for the same purpose. These melt blenders or high speed mixers are ideal for small batch production and their output is in the form of biocomposite pellets. Two- or three-roll mill is another blending technique used for compounding of fibers and matrix prior to injection or compression molding. The fiber—matrix mixture is compounded by passing the mixture through heated counter-rotating rollers to form sheets (Joseph et al., 2005). The compounded biocomposite sheets thus obtained are either hot pressed using a compression molder or shredded into small fragments and injection molded. Yu et al. (2009, 2010) developed ramie and jute fiber- reinforced PLA biocomposites using a two-roll mill prior to hot pressing. The fiber—matrix mixture having varying fiber concentration (10—50%) was blended at 140°C for 5 min using a two-roll mill. Bledzki et al. (2005) conducted a comparative study of different compounding processes prior to compression molding. The fiber—matrix mixture, at a ratio of 1:1, was compounded using a twin-screw extruder (170—195°C for 4 min), a two-roll mill (180°C for 10 min), and high speed mixture (170—175°C and 10—15 mins). Ye et al. (2015) employed a two-roll mill for compounding of sisal fiber-reinforced PLA biocomposites having a fiber mass fraction of 20%. The biocomposites were processed at a temperature of 180°C for 5 min. Okubo et al. (2009) employed a laboratory scale three-roll mill with rollers rotating at varying angular velocities to efficiently disperse microfibrillated cellulose (MFC) (1 and 2 wt%) into a PLA matrix. During compounding the speed of the last roller was fixed at 100 rpm and the PLA/MFC mixture was passed through the rollers 10 times at progressively decreasing gap settings from 70 to 5 p,m.
The interaction between tooling and fiber—matrix mixture is profound during compounding process and strongly influences the behavior of biocomposites. This interaction is observed to be more prominent in case of extrusion and high speed melt blending processes as it results in higher fiber attrition rates. Pultrusion process on the other hand exhibits maximum fiber length retention and is ideal for precompounding long fiber-reinforced pellets.